11.5 A 3.2×1.5×0.8mm3 240nA 1.25-to-5.5V 32kHz-DTCXO RTC module with an overall accuracy of µ1ppm and an all-digital 0.1ppm compensation-resolution scheme at 1Hz

Timekeeping based on a 32kHz XTAL still remains the most popular, cost effective, low power, accurate solution for low-power portable applications. Simple solutions with overall accuracies of a few 100ppm are based on the combination of a through-hole or SMD XTAL together with an oscillator implemented as part of the application SoC (micro-controller, cell phone). As a single-ppm error represents a deviation of 30s/year (nearly 1 hour/year at 100ppm!), temperature-compensated XTAL or MEMS oscillators (TCXO, TCMO) used for time-keeping applications have received significant research attention over the last decade, driven by further miniaturization, tighter accuracy and lower power consumption needs [1–4]. Combining both resonator and oscillator intimately or even better, in a single package, leads to superior stability, improved robustness and lower consumption by minimizing environmental effects (moisture, temperature gradients) and stray capacitance. Real-time clock (RTC) modules integrating further time, timer, calendar, time stamping and alarm functions have become a key power-management block capable of scheduling precise wake-up at user- or pre-defined intervals so that a more complex, energy-constrained application can be heavily duty-cycled and left mostly hibernating (e.g., wireless sensor node). They are found in a variety of consumer, metering, medical, wearable, automotive, communication, outdoor, safety, and automation applications, and are a key component of the upcoming IoE revolution.